Light emitting diodes (LEDs) typically generate a substantially monochromatic light in a specific region of the electromagnetic spectrum. The color of the light output from the LED depends on the material composition of the LED, e.g., InGaN LEDs may produce a blue light whereas AlGaInP LEDs may produce a red light. When it is desired to construct an LED light source that produces a color different from the output color of the LED, it is known to use a wavelength converter to convert all or a portion of the light output from the LED having a first wavelength or wavelength range (the “primary light” or “excitation light”) to light having a second wavelength or wavelength range (the “secondary light” or “emission light”).
Many wavelength converters include a wavelength converting composition that includes a polymeric matrix that contains a dispersion of a wavelength conversion material, which generally functions to convert primary light to secondary light via photoluminescence. Typically, a wavelength-conversion material absorbs relatively high energy primary light, which can excite the wavelength-conversion material to a higher energy state. When the wavelength conversion material returns to a lower energy state it emits secondary light, generally of a different wavelength/wavelength range than the primary light. The wavelength/wavelength range of the secondary light depends on the type of wavelength-conversion material used. Secondary light of a desired wavelength/wavelength range may therefore be attained by proper selection of wavelength conversion material. This process may be understood as “wavelength down conversion.” An LED that is combined with a wavelength converter to produce secondary light may be described as a “wavelength converted LED.”
Conversion of primary light to secondary light also results in the conversion of some portion of incident primary light to heat (Stokes heat). Unless it is dissipated in some manner (e.g., via a heat sink), this heat can accumulate in the wavelength converter. This can cause problems when the wavelength converter is used in wavelength converted LED package. More specifically, heat produced by the wavelength conversion process may degrade the mechanical and optical properties of the wavelength converter, particularly if the converter includes a polymeric matrix. Indeed, the Stokes heat may degrade the properties of the polymeric matrix, potentially leading to premature degradation of the LED package in which the wavelength converter is incorporated. This is particularly true when the polymeric matrix includes an epoxy or silicone based polymer, as such polymers may exhibit significant thermo-oxidative degradation at temperatures exceeding about 90 to about 200° C., depending on the material used.
To address this issue, a lighting designer may limit the amount of primary light that is output by an LED, e.g., by limiting the amount of current applied to an LED package. Controlling the amount of primary light emitted by the package can in turn control the amount of Stokes heat generated during the wavelength conversion process. Although effective, this method artificially limits the amount of primary light that may be output by a LED package, which may be undesirable.